48 BACTERIOLOGY FOR NURSES 



man and be harmless for cattle, and on the con- 

 trary, man is not susceptible to aU the diseases of 

 cattle. 



The tissues of animals and plants which . afford 

 favorable soil for the growth and multiplication of 

 bacteria are known as the hosts, and the host may 

 not always favor bacterial activity ; thus an adult 

 is usually resistant to the so-called children's dis- 

 eases, and on the other hand, hunger, thirst, excessive 

 fatigue, and the wasting diseases all lessen the body's 

 resistance and render the individual susceptible to 

 infections of all kinds. The virulence (ability to 

 produce disease) of the microorganisms also varies, 

 and infection may depend upon the number of bac- 

 teria introduced into the body, and so it may be said 

 that the virulence and number of the invading bac- 

 teria vary quite as much as the susceptibility of the 

 host. 



The channels by which infection may enter the 

 body are (a) by the respiratory tract, (b) by the di- 

 gestive tract, and (c) through the skin. It is 

 thought that 90 per cent of all infections is taken 

 into the body through the nose and mouth, and as 

 we are usually responsible for what goes into our own 

 mouths, it may be seen that it behooves us to observe 

 hygienic personal habits. 



